Original ATK (元（もと）々（もと）の攻（こう）撃（げき）力（りょく）Motomoto no Kōgekiryoku) and original DEF (元（もと）々（もと）の守（しゅ）備（び）力（りょく）Motomoto no Shubiryoku) refer to the ATK and DEF values of a Monster Card before any monster, Spell, or Trap Cards alter the value. They are the ATK and DEF values listed on the card.

Certain monsters only have well-defined original ATK and DEF while they are on the field. For example, when "Chimeratech Overdragon" is Fusion Summoned, its original ATK and DEF become a specific value dependent on the number of Materials used to Summon it. Cards with effects that change their own original ATK/DEF normally have a printed ATK/DEF of ? or 0. Monsters with an original ATK/DEF of ? are not treated as having an ATK/DEF value that is less than or greater than any value; consequently, cards with an original ATK of ? in the hand will not be destroyed by either "Crush Card Virus" or "Deck Devastation Virus". However, such cards are supported by "Catoblepas and the Witch of Fate". If a monster on the field has original ATK/DEF of ? and no effect has changed its ATK/DEF, its ATK/DEF is treated as 0; for example, "The Winged Dragon of Ra - Sphere Mode" has ? ATK and DEF but no effect to change its own ATK or DEF, so they will be treated as 0 on the field by default.